Carbon fiber and C—C composites possess a combination of high strength, high fracture toughness, low density, very high thermal conductivity, and high electrical conductivity. The mechanical strength of carbon fiber and C—C composites actually increases as their operating temperature increases, in sharp contrast to most metals and metallic alloys that become softer and weaker as the temperature increases. This combination of attributes makes carbon fiber and/or C—C composites good candidates for many high temperature applications, such as components used in aerospace heat exchangers and aircraft brake materials.
However, the carbon in carbon fiber and C—C composites tends to oxidize when exposed to air or other oxidizing environments when the temperature exceeds approximately 300° C. When the carbon oxidizes, it loses mass, due to the formation of CO2 and CO gases as oxidation products. This loss in mass leads directly to loss of mechanical strength, as well as loss of integrity, functionality, and ultimately to the failure of the component.
For relatively thick—for instance 1 inch (25 mm) thick—C—C articles, a mass loss of approximately 10-40%, although it is generally not desirable, may sometimes be tolerated before functionality is substantially degraded. In contrast, however, for very thin—for instance single-ply 10-20 mil (0.2-0.5 mm) thick—C—C articles, a five to twenty times smaller carbon mass loss of about 2% may result in an undesirably high 50% loss in tensile strength. Thus, protecting very thin C—C articles from oxidation is quite challenging, in particular because methods that are effective for much thicker articles may not be obviously applicable and/or effective in such very thin articles.
In order to protect the C—C components from oxidization when they are subjected to repeated or sustained high temperatures, barrier coatings are generally applied to the components. Many known barrier coatings, however, tend to develop micro-cracks over time. These micro-cracks allow oxidizing agents to penetrate the coating and reach the underlying C—C composite, resulting in loss of mass and ultimately in component failure.
Known barrier coatings applied to carbon fiber or C—C composite components have failed to protect the carbon component substrate against oxidation due primarily to the difference in thermal expansion coefficients between the specific coating and the C—C component substrate. For example, in a component made from a two-dimensional woven carbon substrate, the component exhibits non-isotropic coefficients of expansion. In an in-plane direction, the thermal expansion coefficient is about −1 ppm/° C. between room temperature and approximately 300° C. and the thermal expansion coefficient increases to about 1 ppm/° C. up to approximately 600° C. In a through-plane direction and where no carbon fibers are present in a 2-dimensional woven component, the coefficient is about 2-4 ppm/° C.
When a barrier or protective coating on a C—C substrate has a thermal expansion coefficient of 3-5 ppm/° C., the coating and substrate portions will expand at differing rates when subjected to elevated temperatures. Micro-cracks will develop over time in the coating, eventually allowing air to penetrate and oxidize the C—C substrate. The result will be a loss of mechanical strength, as well as loss of integrity, functionality, and ultimately failure of the component.
U.S. Pat. No. 6,737,120 B1 relates to carbon fiber or C—C composites useful in a variety of applications. This patent teaches methods of protecting such composites against oxidation by coating them with fluidized-glass type mixtures. The fluidized-glass mixtures are maintained as liquid precursors and are applied to components formed of carbon fiber or C—C composites. Once coated with the precursors, the coated C—C components are heat-treated or annealed for one or more cycles through a series of gradual heating and cooling steps. This process creates solid glass coatings having thicknesses of about 1-10 mils. The thicknesses of the solid glass coatings may be varied by varying the composition of the fluidized glass precursor mixtures, the number of application cycles, and/or the annealing parameters.
U.S. Pat. 6,737,120 B1 teaches that the fluidized glass materials may comprise such materials as borate glasses (boron oxides), phosphate glasses (phosphorus oxides), silicate glasses (silicon oxides), and plumbate glasses (lead oxides). These glasses may include phosphates of manganese, nickel, vanadium, aluminum, and zinc, and/or alkaline and alkaline earth metals such as lithium, sodium, potassium, rubidium, magnesium, and calcium and their oxides, and elemental boron and/or boron compounds such as BN, B4C, B2O3, and H3BO3. By way of example, the patent discloses a boron-containing liquid fluidized glass precursor mixture that includes 29 weight-% phosphoric acid, 2 weight-% manganese phosphate, 3 weight-% potassium hydroxide, 1 weight-% boron nitride, 10 weight-% boron, and 55 weight-% water.
It has been found that the problem of protecting C—C components from oxidizing when subjected to operating temperatures up to about 1100° C. is particularly troublesome when the carbon component is as thin as 3 to 30 mil gauge, and/or complex in shape. Such components may range from flat sheets to fine-dimensioned corrugated fins for use in heat exchanger assemblies. A need exists therefore to prevent oxidation of such thin-gauge carbon fiber or C—C components over their life cycles.